Electron beam vacuum melting furnace



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'1 INVENTOR. 1 lg! WILLIAM D. FIGGINS FIG. 2. WQ/Mf ATTORNEY 3,412,196ELECTRON BEAM VACUUM MELTING FURNACE William D. Figgins, Nashua, N.H.,assignor to Sanders Associates, Inc., Nashua, N.H., a corporation ofDelaware Filed July 13, 1966, Ser. No. 564,957

19 Claims. (Cl. 13-31) ABSTRACT THE DISCLOSURE An electron-ion beamdevice, and more particularly a furnace operating under 'a condition ofpartial vacuum, for heating, melting, welding, or evaporating materials,employing electron beam techniques. A vacuum chamber is filled to a lowpressure with a gas after prior evacuation to remove undesirable vaporsand gases. A high voltage is applied to an electrode to produce a plasmabetween a concave cathode surface and an anode, which anode is used as acrucible. Electrons are extracted from the plasma and mechanicallyfocused by means of a converging focusing cone to produce an intensifiedelectron stream at the crucible.

This invention relates to an electron-ion beam device, and moreparticularly to apparatus for heating, melting, welding, or evaporatingmaterials, employing electron beam techniques. I

It is well known in the art to use high density electron beams to heat,melt, weld, evaporate, etc. materials. The prior art discloses methodsof accomplishing the aboveoutlined objectives by causing a cathode toemit electrons in a controlled atmosphere, which electrons are thendirected toward the material so as to heat the material. Included inthese devices are means for directing and increasing the density of theelectron stream, for example, by employing magnetic, electro-mechanicalor electrostatic focusing. Although these known focusing methods havebeen successful, they are encumbered, in that they are relativelycomplex and costly, usually requiring additional power sources for thefocusing apparatus.

Accordingly, it is an object of this invention to provide an improvedelectron beam device.

Another object of this invention is to provide an improved electron beamvacuum melting furnace.

An additional object of this invention is to provide an electron beamvacuum melting furnace employing mechanical focusing.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following description,taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sketch illustrating a first embodiment of an electron beamvacuum melting furnace employing mechanical focusing; and

FIG. 2 is a sketch of a partial view of the embodiment of FIG. 1,illustrating a modification of the method for providing the mechanicalfocusing.

Briefly, the embodiment illustrated provides an improved method ofmelting materials having high-temperature melting points in an inert gasatmosphere, and comprises a cathode and anode arranged within acontrolled atmosphere, the anode being a crucible in which the substanceto be melted is placed. The cathode-toanode potential is made high,ionizing the inert gas which produces a plasma and causes an electronbeam to be extracted from the plasma. The electrons from the plasma goto the anode and the positive ions go to the cathode. The electron flowis mechanically focused i.e., the electron flow is electron-opticallyfocused by use of a nited States Patent "ice mechanical member, toproduce a high density electron stream at the crucible. The focusingarrangement in one embodiment includes a frusto-conical member made of aheat resistant glass such as that sold under the trademark Pyrex,quartz, or a refractory material having its focal point at the sampletobe melted.

Referring to FIG. 1, there is therein illustrated an embodiment of theinvention. The sample to be melted is placed within a crucible 10, whichis maintained at a high electrical potential relative to a cathode. Thecathode potential could be a large negative value with the anode beingat electrical ground potential. Crucible 10 is located within an outerchamber 11. Crucible 10 in one embodiment, is constructed of copper or asuitable high conduction material, and outer chamber 11 in oneembodiment, is constructed of Pyrex.

Chamber 11 is arranged upon a base plate 12 employing a gasket 13 toprovide a good vacuum seal. Arranged in the upper portion of chamber 11is a cathode member 14 having, in the preferred embodiment-,- a concavesurface 15. It is obvious that the concave surface could be replaced bya straight cathode surface or one of various other shapes. The concavecathode surface is preferred because it offers some degree of focusingin itself. The concave cathode surface has the characteristics of anelectron-optical lens. The focal point of the concave cathode could bearranged at the sample to be melted. The degree of focusing of theconcave cathode could be increased by increasing the cathode-to-anodepotential and/ or decreasing the pressure within chamber 11. Cathodestruct-ure 14 in the embodiment illustrated preferably has a hollowinterior, so that it can be filled with water or other substance forcooling purposes. -The cathode-toouter chamber bonding is alsoaccomplished with a vacuum gasket 13. A beam-focusing cone 16, in thepreferred embodiment having a frusto-conical configuration, is disposedabout cathode member 14 using an O-ring 17. Although in the embodimentsillustrated, the mechanical focusing device is shown as having aconicalconfiguration, this is illustrative only; and other various shapes couldbe employed to provide a converging path for the electron stream. Thefocusing cone is preferably arranged to encompass the cathode surface,as shown in FIG. 1, in order that all the electrons extracted from theplasma enter the cone for focusing onto the anode. Cone 16.is arrangedso that the apex thereof is at or near crucible 10. The beam-focusingcone 16 can be made of Pyrex, fused silica, or a refractory materialhaving suitable insulating and other characteristics. If focusing cone16 is made of Pyrex, fused silica or other quartz material, all of whichcan be made transparent, a desirable feature occurs, in that anexperimenter can view the plasma within cone 16, as well as the samplebeing melted. Cone 16 could also be made of a metal, but this wouldrequire adequate shielding, as well as presenting other problems.

Connected to cathode 14 is an electrode 18 which is coupled to a highvoltage power supply (not shown) by way of an ammeter 19. Pressure ismonitored within the interior of outer chamber 11 by employing athermistor or thermal couple contained within a metering device 21. Theuse of a thermistor or thermal couple is illustrative only, and sundryother devices known in the art could be used to monitor the pressure. Avacuum pump 22 is employed to remove the atmosphere from within chamber11. A gas source 23, which in this embodiment contains argon gas, has aninlet to chamber 11 for supplying gas thereto. The interior of crucible10 has a chamber 24 therein such that water or other liquid coolant canbe passed therethrough via pipes 25 and 26.

The system above described provides a means for melting refractory andother high temperature materials in an inert atmosphere of argon gas.

The sample to be melted is placed within the watercooled crucible 10.The interior of chamber 11 is pumped out by pump 22 such that asubstantial vacuum remains therein. The chamber pressure is usuallyreduced to less than one micron. Argon gas from gas supply 23 is thenadmitted into the chamber, and the pressure therein is adjusted to alevel which is dependent upon the material to be melted. A high voltageis applied to electrode 18, whereby the argon becomes ionized producingan argon plasma; and electrons are extracted from the argon plasma andgo to the anode (crucible). The argon gas is supplied to maintain arelatively constant pressure, and provides purity of the atmosphere aswell as a breakdown path for the electron stream. Pump 22 is maintainedin constant operation to continually withdraw gas from the chamber 11,while gas is continually being supplied to the chamber to maintain it ata constant set pressure. Experimentally, the optimum pressure maintainedin the embodiment illustrated has been in the 10-100 micron range.

Once heating of the substance to be melted begins to occur, asignificant de-gassing takes place; and this gas is withdrawn by pump22. Because the gas within the interior chamber 11 becomes ionized andwould produce argon plasma throughout the chamber, it is difiicult toconfine the electron beam to the area of the crucible unless auxiliarymeans of focusing are used. The concave shape of the cathode surfacewill, in itself, provide some small amount of focusing at the pressuresand voltages used.

One of the significant teachings of this invention is found in the wayin which the electron stream is focused onto crucible 10. Conicallyshaped member 16 is employed to provide a mechanical focusing of theelectron stream, i.e., the electron flow is electron-optically focusedby use of a mechanical member. When a high voltage is applied, via theelectrode 18, between the cathode and the grounded anode 10, an electricfield is established, the lines of which extend between the cathode 15and the anode 10 in a generally frusto-conically shaped bundle ofsubstantially straight lines converging on the anode 10. These electricfield lines terminate substantially at right angles to both anode andcathode. The potential with respect to ground at any point in this fieldis proportional to the distance of the point from the anode as measuredalong the lines. This electric field is always maintained so as toprovide a discharge path for the electrons and ions. The electronssubstantially follow these electric field lines which focus to the anode10, while the ions bombard the concave cathode surface. The cathode thenreleases electrons by secondary electron emission. These electrons causefurther ionization of the argon, resulting in an increased bombardmentof the cathode by returning positive ions. This, in turn, causes furthersecondary emission from the cathode, until that level is reached beyondwhich the electrons and ions will depress the potential fields so as torestrict further flow. Should the electrons deviate from the electricfield lines directed to the anode 10 and hit the inside surface of thefocusing cone 16, secondary emission will result, causing additionalelectrons to be directed to the anode 10.

Without the focusing cone 16, the potential field would partially takethe shape of the concave surface 15, while the outer portion of thepotential field in the fringe area adjacent the interior wall of chamber11 would tend to level off. The potential field would tend to level offmore rapidy with increasing distance from the concave surface 15. Theelectric field lines would be aligned perpendicular to the potentialfield and would fill the entire chamber 11 so as to diffuse the focusingof electrons on anode or crucible 10. Because of the shape of thepotential field, not only would the electric field be partially directedaway from the anode 10, but in addition the momentum of the electronswould be of sufliicient magnitude to carry said electrons past theanode, thereby missing the target.

In addition, when the focusing cone 16 is not used, argon plasma isproduced in the entire inner chamber 11. The sparking and heating thatexists would, in a short period of time, cause the vacuum gasket 13 toloose its sealing characteristics which would cause a degeneration ofthe furnace. When the focusing cone 16 is used, not only will there begreatly improved focusing, but also the ionization of the argon will belimited to the interior of the focusing cone 16. There will be noionization of the argon gas to produce argon plasma in that portion ofthe chamber 11 outside the focusing cone 16. This is because theexternal surface of the dielectric focusing cone 16, is limited tocharging to a voltage below the breakdown voltage of the argon gas.Initially, when the high voltage power supply is turned on, the outsidesurface of focusing cone 16 does charge up but then is bombarded by ionswhile electrons go to the base plate 12. The electrons discharge toground because of the ground connection of base plate 12. The ions causethe exterior surface of focusing cone 16 to become more positive suchthat a voltage less than the breakdown voltage of the argon gas exists.Now, the vacuum gasket will be protected, and of course, improvedmechanical focusing is realized. Focusing cone 16, as shown above,requires no auxiliary sources of electrical power, nor any power at all;rather it is a purely mechanical method requiring no adjustments of anynature.

Referring to FIG. 2, there is illustrated a second mechanical focusingarrangement, comprising conical member 27 and a conical member 28. Sincethe density of the electron stream increases greatly as it approachesthe apex of the conical member, it causes said apex to heat up to a veryhigh temperature which, in some applications, could cause the bottom ofthe cone to melt. Thus, there is provided a shortened cone 27 whichcould be constructed, for example, of Pyrex as above, and a secondconical element 28 at the bottom thereof which could be constructed offused silica or refractory material which could take the increasedheating with little deteriorating effect. It would also be possible toProvide a single cone having a plural composition. That is, the upperportion thereof could be made of Pyrex, and the lower portion of fusedsilica or other material having a relatively high melting point.

Of course, as mentioned above, the entire cone could be constructed offused silica or a refractory material, but this is very costly and notnecessary, in view of the embodiment illustrated in FIG. 2. Amodification of the focusing cones discussed would be to cool the cones,for example, by making the walls of the cones hollow and causing acooling fluid to flow therein.

Although the embodiments illustrated are directed toward an electronbeam vacuum melting furnace, the principles of the invention could beapplied to any arrangement in which electron beam focusing is required,for example, in vacuum tube technology, or a cathode ray tube in whichno scan is required. It would also be possible to use the mechanicalfocusing to focus the electron stream down to a much finer point than isillustrated in the figures, and provide means for moving the beam focuscone, and use the focused stream for thin film machining, or otheretching process. It would also be possible to use the mechanicallyfocused beam to heat up a material to provide an ultraviolet or infraredlight source, for example, for satellite tracking operations.

Thus, it is to be understood that the embodiments shown are illustrativeonly, and that many variations and modifications may be made withoutdeparting from the principles of the invention herein disclosed anddefined by the appended claims.

I claim:

1. An electron-ion beam device, comprising:

means for generating an electron stream, said means comprising:

a cathode,

an anode, and

means including said cathode and anode for maintaining a plasmatherebetween, and

means mechanically focusing said electron stream into a path convergingon said anode.

2. Apparatus as in claim 1, in which said cathode has a concave surface.

3. Apparatus as in claim 2, in which the focal point of said concavecathode is at said anode.

4. An electron beam device as in claim 1, for heating a substance, inwhich said anode is a crucible in which the substance to be heated isplaced.

5. Apparatus as in claim 4, further including means for cooling saidcrucible.

6. Apparatus as in claim 1, in which said means for generating anelectron stream includes means for maintaining a high voltage betweensaid cathode and said anode.

7. Apparatus as in claim 6, in which said cathode has a hollow interiorconfiguration, further including a cooling fluid disposed within saidhollow interior.

8. Apparatus as in claim 1, in which said mechanically focusing meansincludes a member having a continuously decreasing cross-sectional area.

9. Apparatus as in claim 8, in which said member comprises a heatresistant glass.

10. Apparatus as in claim 8, in which said member comprises a refractorymaterial composition.

11. Apparatus as in claim 8, in which said member comprises a quartzcomposition.

12. Apparatus as in claim 8, in which said member has a conicalconfiguration.

13. Apparatus as in claim 1, in which said mechanicalfocusing' meansincludes a first frusto-conically shaped member having its apex directedtoward said anode, and a second member having a portion thereof of afrustoconical confiuration also having its apex directed toward saidanode, said second member being disposed between said first member andsaid anode.

14. Apparatus as in claim 13, in which the diameter across the apex ofsaid second frusto-conical member is less than the diameter across theapex of said first frustoconical member.

15. Apparatus as in claim 14, in which said second frusto-conical membercomprises a high-temperature, non-electrically conductive materialcomposition.

16. Apparatus as in claim 1, in which said mechanically focusing meansis positioned intermediate said cathode and anode.

17. Apparatus as in claim 1, in which said means for maintaining aplasma therebetween further comprises:

walls defining a chamber,

said chamber containing a gas at less than atmospheric pressure, and

means for maintaining a high voltage between said cathode and anode.

18. Apparatus as in claim 17, in which said gas is argon.

19. An electron-ion beam device, comprising:

means for generating an electron stream, said means comprising:

a cathode,

an anode, and

means including said cathode and anode for creating a plasmatherebetween, and

means for focusing said electron stream on said anode,

said means comprising a member composed of dielectric material andshaped to define a converging path to said anode.

References Cited UNITED STATES PATENTS 2,793,282 5/ 1957 Steigerwald.2,899,556 8/1959 Schopper et al.

848,600 3/ 1907 Von Pirani. 3,192,318 6/1965 Schleich et a1. 3,311,7463/ 1967 Linstrom 250-495 3,202,794 8/ 1965 Shrader et a1 1331 XR ROBERTSCHAEFER, Primary Examiner.

M. GINSBURG, Assistant Examiner.

